I have a 2004 dodge ram 1500 4x4 with the 5.7L engine. I bought this truck used and after driving it for a while noticed the engine temp would go up and down and never fully warm up. I replaced the thermostat and now it sits just below center once warmed up, so I assume the thermostat was stuck open. Last winter the heater would barley make enough heat to keep the truck warm on cold days. It will make ok heat on the second fan setting but just luke warm air on the fastest fan setting. I assuming whatever was in the coolant that plugged the thermostat also plugged up the heater core and now am not getting a full passage of coolant to heat the air. Any suggestion to repair this or am I on the right track? The problem hasn't got any worse and no leaking coolant anywhere. Just seems like restricted flow.

 

There's been some post here, in the 3rd Gen forum, about about vent doors sticking open...?? not sure if it covers your 04. but you might do a search on that.

I'm sure some one else will chime in soon

 

I have an 02 1500 v-8 4.7L 2WD. last year I had the same issue with the heater. The heater would only operate at desired temps when the truck was in motion. When at idle the heater blew cool air. Occasionaly I could turn the fan off then back on and it would reset. However this year the heater will only attemp to blow desired heat when fan is initially turned on then as time passes the temp turn cooler again. I have had coolant flushed, new radiator cap, removed dash and checked the recirc door and motor, then blend door and motor, and Mode 1 axle and motor. All has checked out fine. I have same temps on the in and out hose from the heater core and no visible or hint of coolant smell from the heater core. I have no loss of coolant in the overflow as well. I have not changed the thermostat since I have not had any problems with it operating visually on the temp gauge or while truck is operating at speed. I have heard it may be a vacuum leak. I will post again the further I research the issue. If anyone has found a soulution please share your info. Thanks